1. Field of the Invention
The present invention relates to a liquid crystal display device and a manufacturing method thereof.
2. Description of the Related Art
Conventionally, a liquid crystal display device, which uses a chiral nematic composition that is made to exhibit a cholesteric phase at room temperature by applying a chiral agent to a nematic liquid crystal, has been known. Such a liquid crystal display device basically has a structure in which a chiral nematic liquid crystal composition is sandwiched between a pair of substrates having transparent electrodes. A high or low pulse voltage (driving voltage) is applied between the electrodes to switch the liquid crystal between a planar (PL) state and a focal conic (FC) state or a homeotropic state so as to carry out a displaying operation. In particular, in the PL state, light having a specific peak wavelength is selectively reflected.
In such a display device, in an attempt to carry out a black and white display and a wide-viewing-angle display, a technique has been proposed in which: a monomer and a polymerization initiator are contained in the nematic liquid crystal composition and after the display device has been once manufactured, the display device is subjected to an ultraviolet-ray (UV) irradiation or the like so that the polymer is polymerized (for example, Non-Patent Document 1).                [Non-Patent Document 1] “SID 97 DIGEST”, p. 101 to 104, written by R. Q. Ma and one other.        
In the above-mentioned technique, however, even when a polymerizing process is carried out sufficiently, it is not possible to completely carry out the polymerizing process so as not to cause residual unreacted monomers; therefore, upon use of the device, the residual unreacted monomers gradually react to ultraviolet rays to cause a problem in that the device is changed in its display performance. In other words, the display color is changed, and the contrast is lowered between the PL state and the FC state or the Homeo state. In particular, in the case when a black and while display system that displays white in the PL state, and displays black in the FC state or the Homeo state is carried out, the above-mentioned problem becomes prominent.
With respect to the method for improving the contrast, a method, which improves a whiteness degree at the time of displaying white by increasing the reflectance in a comparatively wide wavelength in the PL state, has been known. In such a method, however, it is difficult to select a proper material used for increasing the reflectance in the comparatively wide wavelength in the PL state, and it is necessary to increase the cell gap so as to increase the reflectance in the wide wavelength, with the result that another problem is raised from the viewpoint of a driving voltage.